1. Field of the Invention
The present invention relates to single cylinder internal combustion engines and, more particularly, to a safety device for single cylinder internal combustion engines whereby the engine is stopped within a predetermined time period.
2. Description of the Prior Art
Single cylinder internal combustion engines are generally utilized in most walk-behind lawn mowers. In these lawn mowers, the single cylinder internal combustion engine includes a flywheel disposed on one end of a crankshaft along with a blade disposed on the end opposite the flywheel. The lawn mower has a cover or deck disposed between the blade and the engine with the crankshaft extending therethrough. The cover surrounds the rotating blade keeping grass and debris from spewing out in all directions. A discharge opening located in one side lets the grass and debris exit onto the lawn or into a grass catching bag. Within the cover, the blade is rotated at a high velocity by the crankshaft in order to effect efficient cutting of the grass. Because of the high velocity at which the blade is rotating, as well as the sharpness of the blade, a lawn mower, like all power tools and machines, has an inherent potential for risk.
Because of this, safety standards have been promulgated which require that the engine, and thus the rotating blade, stop when the operator is not using the lawn mower, steps away from, or tries to leave the mower unattended. Such devices for stopping the engine when the operator is not utilizing the lawn mower or in attendance, have come to be known as "deadman" controls. These deadman controls typically consist of two parts, a positive braking mechanism disposed within the engine, and a lever disposed at the upper handle portion connected by a cable to the braking mechanism for actuating the same. The deadman lever is normally biased into a position such that the engine will not start until the operator positively moves or biases the lever opposite the normal biased position. Opposite biasing is accomplished when the operator holds the lever against the upper handle while pushing the lawn mower. In order to keep the engine running, the deadman lever must remain biased by the operator. Once the lever is released, the natural bias of the system actuates the braking mechanism and the engine stops.
In some prior art systems, the positive braking mechanism is accomplished by a flywheel brake. The flywheel brake consists of a brake shoe with a pad normally positively biased against the rim of the flywheel, either the outside or inside, to stop the flywheel by biased friction when the deadman lever is released. By stopping the flywheel, the crankshaft and associated piston also stop thereby stopping the engine altogether.
Such systems are described in U.S. Pat. Nos. 4,889,213 issued Dec. 26, 1989, entitled "Compliance Brake For An Internal Combustion Engine Powered Implement," and 4,979,596 issued Dec. 25, 1990, entitled "Safety Brake For An Engine," by inventor Lee E. Roller, both of which are specifically incorporated herein by reference.
In addition to or separate from the flywheel brake mechanism, prior art systems ground or short out the primary coil of the ignition transformer so that no current will flow into the primary coil.
Also known in the prior art are exhaust retard mechanisms that work in conjunction with the engine piston in the combustion chamber. One such system is a "Jake" brake that utilizes the pressure in the combustion chamber to slow the engine. It is believed that camshaft timing is varied in order to increase the pumping action as the exhaust leaves the cylinders, thus increasing the retarding force on the pistons and connected mechanisms. U.S. Pat. No. 3,023,870 Udelman issued Mar. 6, 1962, teaches an auxiliary brake which includes a manually operated ignition device so the engine acts as a brake. The point of ignition is advanced, thus opening the exhaust valve when the piston is three-quarters on its compression stroke.
It is an object of the present invention to provide a more reliable and efficient engine brake safety device.